In industrial high frequency (HF) plasma technology, matching the electrical impedance of the high frequency power source (e.g., a high frequency power generator) to the consumer, in particular a plasma discharge, is an important detail. Normally, a matching circuit undertakes this task. In the process, electrical reactances form a transformation circuit, which transforms the complex impedance of the load (often very low-ohm) into that of the high frequency power generator (normally (50+0j)Ω). Known embodiments are circuits consisting of two capacitors and one coil.
Since the impedance of the consumer (the load) is constantly changing as a result of the inconsistent property of the plasma, matching circuits are provided with a control, which normally readjusts two of the three reactances such that the impedance detected by the high frequency power generator remains constant.
Matching circuits have the disadvantage, however, that their variable reactances—often stepper-motor-controlled rotary vacuum capacitors for higher high frequency power—can only be readjusted slowly (in the millisecond to second range) and, moreover, are subject to wear.
To be able to react on a faster timescale (micro to millisecond), control of the excitation frequency of the high frequency power generator has established itself in the industrial use of plasma. In the process, the frequency of the fundamental wave (synonymous with fundamental frequency) (normally from 2 MHz to 160 MHz) changes such that improved matching of the impedance of the high frequency power generator to the load emerges.
One disadvantage of frequency control is the limited scope of influence as a result of low dependence of the impedance on the frequency and the often limited frequency range of the high frequency power generator. A further disadvantage of frequency control is the inherent impossibility of being able to balance every impedance value of the load since the frequency axis is only a one-dimensional value, but two linearly independent parameters are required for transformation of a complex and therefore two-dimensional impedance.
For these reasons, a controlled matching circuit is usually required in addition. While the frequency control balances the rapid plasma changes, a matching circuit can transform a wide, two-dimensional impedance range and therefore undertake the basic impedance matching.
The use of two independent controls is complicated. At best, the overall gain of the two control circuits forms a plateau, the controls remain stable at each found position. Even low tolerances such as offset voltages or rounding errors in digital intermediate calculations can lead to the controls building each other up, which is often stopped only by reaching a control limit. For example, the readjustment of the frequency can lead to the matching circuit adjusting its reactances which in turn leads to a renewed readjustment of the frequency, etc. until one of the reactances cannot be readjusted any further or the limit of the frequency range is reached. It is therefore difficult to reach a stable state for the impedance matching.